Multigraph is an open source project. Anyone is welcome to download the source code and modify it. You can download the source code from http://www.multigraph.org/download.
Multigraph is written in ActionScript, which is the language of the Adobe Flash player. The Multigraph source code is organized as a project for the Flex Builder Eclipse (http://www.eclipse.org) plugin from Adobe, which is available from http://www.adobe.com/products/flex. The Multigraph source code zip file includes the Eclipse project files, so importing the project into your copy of Eclipse should be straightforward. The Flex Builder plugin is unfortunately not free, but you can download a free 60-day trial version from the above web page. Also, it IS free for use in academic institutions (https://freeriatools.adobe.com/flex), and there is an alpha version for Linux that is free to anyone (http://labs.adobe.com/technologies/flex/flexbuilder_linux).
In addition to Flex Builder, it is also possible to compile Multigraph with Adobe's free SDK compiler, which is available from http://www.adobe.com/products/flex/flexdownloads/index.html. If you are going to do much work with the Multigraph source code, though, you will probably want to get Flex Builder.
If you have any questions about working with the Multigraph source code, send an email to the multigraph-users mailing list, which is described in Obtaining Support / Mailing List.